1. Field of the Invention
The present invention relates to an exhaust emission purifying apparatus for an engine for reducing and eliminating nitrogen oxides (NOx) which is emitted from a diesel engine, a gasoline engine, or the like mounted on a mobile vehicle, by supplying a reducing agent into an exhaust emission on an upstream side of a reduction catalyst, and particularly, relates to an exhaust emission purifying apparatus for an engine for suppressing deposition of a constituent of the reducing agent (solute of the reducing agent) in an exhaust passage to enhance an elimination rate of NOx even if an exhaust temperature from the engine is low and for detecting deposition and accumulation of the constituent of the reducing agent in the exhaust passage.
2. Description of the Related Art
As a system for eliminating especially NOx out of particulate matter (PM) in exhaust emission emitted from the engine to purify the exhaust emission, there are proposed some exhaust emission purifying apparatuses. In such an exhaust emission purifying apparatus, a reduction catalyst is disposed in an exhaust system of the engine, a reducing agent is injection-supplied to an exhaust passage on an upstream side of the reduction catalyst, and NOx in the exhaust emission is catalytic reduced by the reducing agent and is subjected to a process of purification to be converted into harmless components. The reducing agent is stored in a liquid state at room temperature in a storage tank and a required amount thereof is injection-supplied from an injection nozzle. For the reduction reaction, ammonia with satisfactory reactivity with NOx is used. As the reducing agent, a urea aqueous solution, an ammonia aqueous solution, and other reducing agent aqueous solutions which hydrolyze to easily generate ammonia are used (see Japanese Unexamined Patent Application No. 2000-27627 and “Catalysts & Catalysis” Vol. 45 No. 3 2003, Catalysis Society of Japan, published on Apr. 10, 2003, page 227, for example).
However, in the above conventional exhaust emission purifying apparatus, when an exhaust temperature from the engine is lower than 132° C., for example, there is a possibility that urea in the urea aqueous solution (hereafter referred to as “urea water”) as the reducing agent injection-supplied into an exhaust emission in an exhaust passage is precipitated or deposited and adheres on an inner wall surface of an exhaust pipe. Namely, since the urea water condensates at about 100° C. and a melting point of urea is 132° C., the urea remains precipitated in a temperature range in which the exhaust emission exhausted from the engine is equal to or higher than 100° C. at which the urea water condensates but lower than 132° C. which is the melting point of urea. In this case, the urea water is reduced by an amount corresponding to the precipitated urea, the urea water injection-supplied from the injection nozzle is not effectively used for the catalytic reduction reaction, there is a possibility that NOx cannot satisfactorily be reduced and eliminated, and an elimination rate of NOx might decrease in some cases.
Moreover, when only moisture evaporates from the urea water injection-supplied into the exhaust emission on the upstream side of the reduction catalyst and urea is precipitated and accumulates in the exhaust pipe in the vicinity of the injection nozzle, the accumulated urea forms obstruction against flowing of the exhaust emission in the exhaust pipe. Therefore, there might be possibility of increase in exhaust pressure and decrease of mileage. When diesel oil, gasoline, alcohol, or the like is used as a reducing agent, there may be a similar possibility that a constituent such as a carbon is precipitated and accumulates in the exhaust pipe.